You'll need the Mote control board to power the Mote sticks, but this can be powered by USB although I'd be wary of the power draw on them if you are powering a Picade too. Maybe if it had a powered USB hub, then I can't see why it wouldn't work? Might be a bit overkill though - maybe use a Blinkt strip?

Basically, I'd like to know if I can get a light strip that constantly shines one colour (probably white) when I power up the Picade. I don't really want to use a second power supply (My Pi 3 is powered by 1 power supply) - wonder if I could just plug something into a spare USB port on the Pi.

yes, you can... Mote hooks up to USB. However, as @Mr_A pointed out, if the power draw of the whole thing outweighs your PSU, you will run into trouble.

I guess the Mote has the advantage that if you hit a wall it is specifically designed to be powered independently, if needs be. It's also modular - up to 4x 16 pixels sticks.

On the flip side, a blinkt will draw less, not least because we're comparing 8 pixels to 16 (minimum) in the case of Mote (although technically you could cut a Mote stick to any length if you really wanted to).

note that blinkt! hooks itself up to the Pi GPIO, so if you were looking at further GPIO based add-ons, this could also turn out not to be such a good idea, although that will be by far the cheapest option (£5 vs min £22).

If I were you I probably would go with the Blinkt in the first instance to test the water, but the installation will be a bit more fiddly.

In regards to Mote and its power draw, how can I calculate its power needs?

I will be using a Pi3 which will be powered by an official Raspberry Pi power supply. Then I guess I have to consider Picade's power thirst? I haven't a magoo when it comes to amp watt talk, sorry (embarrassed).

Thanks @archieroques I did go down this path. A strip of RBG LEDs which plugs into the Pi. It has brightness control (about 10 levels!) So, anything past level 2 starts to cause problems with the Pi. I have it on level 1 brightness and it's been on for a few hours with no issues. The brightness is fine too.

If over USB, you could use another power supply (like a powered USB hub) to provide separate power to go higher power if you wanted. You might be able to use this micro USB breakout or this Sparkfun hydra power cable for no soldering to provide power to the strips if they connect to the GPIO.

With the method you're using, the power is being pulled through the Raspberry Pi - so the current is limited (as I believe the Pi has current limiting on USB ports, as most PCs do (see this post for more info)) whereas if you connect it directly to the power supply, you can draw as much current as the PSU can supply without putting strain on the Pi.

Yeah seems using a second power supply would be the best solution. Just thought I'd see if I do this via the Pi so I only had one psu connected. Two poorer supplies, for me, is a bit less dynamic and messy if you get my drift?